Page 50 - Sedimentology and Stratigraphy

Evaporite Minerals 37

either as gypsum or anhydrite. Calcium sulphate is
precipitated from seawater once evaporation has con-
centrated the water to 19% of its original volume.
Gypsum is the hydrous form of the mineral
(CaSO 4 .2H 2 O). It precipitates at the surface under all
but the most arid conditions but may become dehy-
drated to anhydrite on burial (18.5). Anhydrite has
no water in the crystal structure (CaSO 4 ) and forms
either by direct precipitation in arid shorelines
(15.2.3) or as a result of alteration of gypsum by
burial. It may become hydrated to gypsum if water
is introduced. Primary gypsum occurs as elongate
crystals of selenite when it forms from precipitation
out of water. If it forms as a result of the rehydration
Fig. 3.10 White halite precipitated on the shores of the
of anhydrite it has a fine crystalline form in nodules of Dead Sea, Jordan, which has a higher concentration of ions
alabaster. Gypsum also occurs as a fibrous form in than normal seawater.
secondary veins.
Gypsum is readily distinguished from calcium car- chloride mineral has a more bitter taste than ‘normal
bonate minerals in the field because it is softer (hard- salt’ and is much less common. Halite is soft (hardness
ness 2, easily scratched with a fingernail) and does 2.5, slightly more than gypsum but still scratched by
not react with dilute HCl: it can be distinguished from a fingernail), white or colourless. In thin-section
halite by the fact that it does not taste salty. Crystals of halite crystals may show a strong cleavage with
gypsum have a low relief when they are viewed under planes at right angles and, being a cubic mineral, it
the microscope, cleavage is usually well developed is isotropic.
and the birefringence colours are low-order greys.
Anhydrite is a harder (hardness 3.5), denser mineral
than gypsum: it is commonly white in hand specimen, 3.2.3 Other evaporite minerals
and is not easily scratched by a fingernail. In thin-
section the high density means crystals have a rela- Evaporation of seawater can yield other minerals,
tively high relief; birefringence colours are moderate, which are rarely found in large amounts but can be
higher-order colours than gypsum. economically important. In particular, potassium
chloride, sylvite (KCl), is an important source of
industrial potash that occurs associated with halite
3.2.2 Halite and is interpreted as the product of extreme evapora-
tion of marine waters. However, evaporation of mod-
Halite (NaCl) precipitates out of seawater once it has ern waters results in a number of different magnesium
been concentrated to 9.5% of its original volume sulphate (MgSO 4 ) minerals rather than sylvite, and
(Fig. 3.10). It may occur as thick crystalline beds or this has led to suggestions that the chemical composi-
as individual crystals that have a distinctive cubic tion of seawater has not been constant over hundreds
symmetry, sometimes with a stepped crystal face of millions of years (Hardie 1996). Variations in the
(a hopper crystal). The high solubility of sodium relative importance of meteoric waters (run-off from
chloride means that it is only preserved in rocks in land) and hydrothermal waters (from mid-ocean ridge
the absence of dilute groundwater, which would dis- vents) are thought to be the reason for these varia-
solve it. Surface exposures of halite can be found in tions in water chemistry, which either favour KCl or
some arid regions where it is not removed by rain- MgSO 4 precipitation at different times.
water. Saline lakes (10.3) generally contain the same dis-
Naturally occurring halite is rock salt, so the sim- solved ions as seawater, but the proportions are
plest test to confirm the presence of the mineral usually different, and this results in suites of evap-
is taste: the only mineral it might be confused with orite minerals characteristic of different lake chem-
on this basis is sylvite (below), but this potassium istries. Most of these minerals are sulphates,